This is quoted from Winexpert and it applies to Beer yeast as well as Wine yeast:

Q: What is the best way to handle the yeast?

A: If you look at the instructions in your wine kit (and please, do), they will likely instruct you to sprinkle your packet of yeast directly on to the must. Yet if you read the yeast package (and many winemaking textbooks) they recommend rehydrating the yeast. If the objective is to deliver the maximum number of yeast cells to the must, which technique is best?

It turns out that the answer is not as simple as one or the other, but the main point is that rehydration is not really necessary. You can rehydrate your yeast if you absolutely want to, but be sure to do it accurately and precisely, as explained further below. The rest of us will tear open the package and dump it in, and spend the extra time sampling our last batch!

When performed correctly, rehydrating gives the highest live cell counts, and the quickest, most thorough fermentation. The catch is, it has to be done precisely correctly. Lalvin EC 1118 champagne yeast, for instance, asks you to add the yeast to 10 times its weight in water at 40-43°C (104-109°F).

Breaking it down, the amount of '10 times' is important if you're trying to maximise live cell counts. That's because the yeast is dried on a substrate of nutrients and sugars. At a ratio of 10:1 water/yeast, the osmotic pressure allows for maximum nutrient uptake (osmotic pressure is influenced by the dissolved solids in the water, like nutrients and sugars). If too much water is used, the yeast will grow only sluggishly. If too little water is used, the cells may burst from the flood of liquid and nutrients forced into them.

Secondly, the temperature range is inflexible. The outer integument of a yeast cell is made up of two layers of fatty acids. These layers soften best in warm water, much as greasy film will come off of dishes best in warm water. Once it has softened up, it will allow the passage of nutrients and waste products in and out of the cell much more efficiently. If the water isn't warm enough, the cell won't soften. If it's too warm, generally anywhere above 52°C (125.6°F) the yeast cell will cook and die.

The next thing you have to worry about is temperature shear. Yeast is terrifically sensitive to environmental conditions. If it goes too quickly from a favourable temperature to a less favourable one, weakened cells may die, and others may go dormant, in an attempt to ride out the temperature shift. This reduces the numbers of live, viable cells available to ferment the must, and gives spoilage organisms a chance to get a foothold, and potentially ruin your wine. So if you are rehydrating your yeast, you'll have to wait as the yeast cools to within two degrees of your must temperature before adding it: accuracy counts!

On the other hand, simply dumping the yeast onto the top of the must should result in lower cell counts. Empirical evidence shows this isn't the case: the yeast appear to know what they're doing. Generally, a five-gram packet of yeast will have less than a six-hour lag phase on an average wine kit. This is perfectly acceptable, and isn't long enough to allow spoilage organisms to get a foothold in your wine. Plus, it's a heck of a lot simpler than going through the rehydrating process, fraught as it is with risks.